As explained in said parent application, in the prior art, three types of compensation have been proposed. Fixed compensation involved using a capillary or orifice to act as a fixed value resistance. Variable compensation typically constituted the use of a diaphragm and/or valves to provide a flow inversely proportional to the pocket resistance, thereby creating a larger pressure differential than created with the use of a fixed compensation device. Both of these types of compensation, however, must be tuned to the bearing gap and require the manufacture and installation of additional mechanical components subject to clogging and manufacturing errors.
As smaller and smaller bearing gaps are sought in order to increase the performance of the bearing, manufacturing errors make the use of either of these types of compensation more and more difficult by requiring hand-tuning of the compensation device. For high speed machines where a bearing failure (e.g., due to a clog or an improperly hand-tuned bearing) could result in a very non-optimal machine operating condition, there has been a great reluctance to use conventional hydrostatic bearings.
In addition, there is a growing concern about environmental issues, both in terms of pollution, human skin dermatitis, and heat generated by the bearing which causes thermal growth errors in the machine. All these issues are a direct result of using oil in hydrostatic bearings. If a more environmentally friendly fluid, such as water, could be used, then there would not be a disposal problem with the fluid, and working with the machine (e.g., assembly and testing and service) would not result in dermatitis problems for workers. Fire hazards would also be virtually eliminated, as would thermal growth errors. Furthermore, since many processes now use water-based coolants, there would be less of a chance for machine or process degradation if some of the process control fluid leaked into the bearing fluid or vice versa. In addition, one of the most common types of bearing failure may be avoided, which is the wear of rolling element bearings caused by a cleaner blowing dirt off the machine and invariably causing some of the dirt to be blown by the bearing seals and into the bearings. A fluidstatic bearing, on the other hand, upon startup, will clean itself.
The solution is not so simple, however, as merely replacing the oil with water in a conventional bearing design. Firstly, because water has one-tenth the viscosity of a typical light hydrostatic oil (e.g., ISO 10 oil), ten times the flow will result if the water is used in the same bearing; and in higher speed applications (e.g., spindles) turbulence will result causing substantially higher rates of heat generation than occur with the use of oil.
Since the flow is proportional to the cube of the bearing gap, obtaining a flow rate with water that is equivalent to that of the oil bearing, requires that the water bearing have about one-half of the gap of the oil bearing. The Reynolds number, which is an indication of turbulence, is proportional to the bearing gap. Fortunately, the four-times greater heat capacity of water (as compared with a typical hydrostatic oil) means that by reducing the gap by one-half or more, allows for minimal heat generation and the avoidance of turbulence in moderate speed applications optimal, for example, for large grinding machines and some machine tool spindles. Other applications such as lapping and polishing machines, run at relatively low speeds; but they require extreme accuracy in the presence of extremely adverse conditions (e.g., the caustic solution used to polish wafers for integrated circuit production, for example, or for production of memory disc substrates and the like).
A third type of compensation, as also described in said parent application, is called self-compensation because it uses the change in bearing gap to allow the bearing to change the flow of fluid to the pockets, by itself. Existing self compensation methods have utilized linear passageways on the face of the bearing and have been directed primarily to applications in spindles, as later more fully discussed. These designs have not, however, proven themselves to provide acceptable performance in the commercial sector because of inefficient flow patterns that are difficult analytically to determine, particularly the flow field near the end of the linear grooves, often resulting in improper resistance design and which then require hand-tuning of the compensator. Difficulty has also been experienced with prior linear groove self-compensation units because the geometry has not always been realistically implementable or deterministic enough to allow for easy design and build-with-confidence-that-it-will-work scenarios. In addition, methods need to be developed to introduce the fluid into the bearing to prevent the occurrence of turbulence which would cause unacceptable increases in heat generation.
Underlying the present invention, is the discovery that through the use of the general type of bearing construction of the parent invention, modified for shaft or spindle applications of the present invention, and embodying self-compensating units in the form of a pressurized annulus that feeds the fluid to a hole in its center which is then connected to a bearing pocket at an opposing region of the shaft via a hole (that for high speed motion applications of the present invention preferably intersects the pocket at an angle such that the fluid that enters the pocket flows into the pocket with a principal component (tangential) in the direction of motion), the limitations of such prior art approaches are admirably eliminated. The annulus is easy to manufacture and is more structurally stable than linear passages; and, furthermore, the fluid flow from the circular annulus to the center feed hole can be analytically determined with great accuracy. The intersection angle of the feeder hole to the pocket can be found by using stream functions to visualize the flow as later explained. When flow velocity reversal is avoided, the chance for occurrence of turbulence can be minimized. Typically the appropriate angle is from 30 to 60 degrees. For bi-directional bearings, two fluid paths to two feed holes, switched by a reciprocating valve, may be used as later more fully discussed.